It is not possible to call a variadic function in a standard conforming (i.e. portable) way.
You could make the definition of the C function take two parameters only (so it is no longer variadic - existing references to the function would need to be modified), with the second parameter in the C function a pointer, either NULL to indicate that there are no additional things being passed, or pointing at an array of pointers (perhaps NULL terminated) or whatever, with any additional things. In F201X an interface body for this sort of function might be able to use the OPTIONAL attribute for the second argument.